1. Field of the Invention
The present invention relates to an electronic ignition device for an internal combustion engine which is used in ignition of an internal combustion engine such as an automobile.
2. Description of the Related Art
A conventional ignition device for an internal combustion engine is configured in circuit in such a way that a gate of a gate drive type power device (power device) is connected to a power source through a resistor provided outside an integrated circuit (IC), and is pulled up to drive the power device, while a gate current is sunk (pulled down) by an NPN type transistor to turn OFF the power source. In addition, a high voltage cut-off circuit for preventing the power device from being broken down due to the application of an over-voltage is provided in the ignition device for an internal combustion engine.
FIG. 11 is a circuit diagram, partly in block diagram, showing a configuration of a conventional ignition device for an internal combustion engine. Referring now to FIG. 11, an output terminal of a controller (ECU) 1 is connected to a waveform shaping circuit 2, and an output terminal of the waveform shaping circuit 2 is in turn connected to a base terminal of an NPN type transistor 3. A collector terminal of the NPN type transistor 3 is connected to both of a power source pull-up resistor 12 and a terminal (1) of a drive circuit 4.
A terminal (2) of the drive circuit 4 is connected to a gate of a power device (a gate drive type power device) 5 (an IGBT in this case), and a power source 6 is connected to a coil 7. In addition, a power source pull-up resistor 13 provided outside the IC is connected to a terminal (3) of the drive circuit 4.
In addition, a filter circuit constituted by a resistor 9 and a capacitor 10 which are both provided outside the IC is connected to a terminal (4) of a high voltage cut-off circuit 11 for protecting the power device from being broken down due to the load dump.
The high voltage side of the coil 7 is grounded (connected to the ground) through an ignition plug 8. An output terminal (5) of the high voltage cut-off circuit is connected to a output terminal of the waveform shaping circuit 2.
Next, an internal configuration of the drive circuit 4 will hereinbelow be described with reference to FIG. 12. FIG. 12 is a circuit diagram showing an internal configuration of the drive circuit 4 of a conventional ignition device for an internal combustion engine.
Referring now to FIG. 12, the drive circuit 4 includes an NPN type transistor 14 for driving the power device 5, a resistor 15 connected to a gate of the power device 5, and a clamping diode 16 for protecting the power device from suffering the surge. The clamping diode 16 is connected between a collector of the NPN type transistor 14 and the ground (GND).
Next, an internal configuration of the high voltage cut-off circuit 11 will hereinbelow be described with reference to FIG. 13. FIG. 13 is a circuit diagram showing an internal configuration of the high voltage cut-off circuit 11 of a conventional ignition device for an internal combustion engine.
Referring now to FIG. 13, the high voltage cut-off circuit 11 includes a Zener diode 30 and resistors 34 and 35 acting as a circuit for detecting a power source voltage, and a transistor 36 for cutting off a primary current on the basis of the detected power source voltage. By the way, the circuit constituted by the Zener diode 30, the transistors 31 and 33, and the resistor 32 is the circuit for clamping the power source voltage.
Next, the operation of the conventional ignition device for an internal combustion engine will hereinbelow be described with reference to FIG. 14. FIG. 14 is a waveform chart useful in explaining the operation of portions of the conventional ignition device for an internal combustion engine.
Referring now to FIGS. 11, 12 and 14, an ignition signal (a) which has been outputted from the controller 1 is inputted to the waveform shaping circuit 2. Then, the waveform shaping circuit 2 supplies a switching signal (c) to the base terminal of the NPN type transistor 3 to drive the transistor 3.
Subsequently, a switching signal (d) is inputted to the drive circuit 4 through the transistor 3 and the power source pull-up resistor 12.
The NPN type transistor 14 in the drive circuit 4 is switched in accordance with a switching signal (d) which has been inputted to the drive circuit 4 to pull up a current from the power source pull-up resistor 13 mounted outside the IC, thereby outputting a switching signal (e) used to drive the power device 5.
A coil primary current (f) flowing through a primary winding of the coil 7 is caused to flow synchronously with the gate voltage. Then, when this coil primary current (f) is cut off, the voltage is supplied to the ignition plug 8 on the basis of the high voltage generated through a secondary winding of the coil 7 to ignite the ignition plug 8, thereby driving the internal combustion engine.
Note that, the NPN type transistor 14 pulls up a current a quantity of which is determined on the basis of the voltage value of the power source 6 and the resistance value of the power source pull-up resistor 13. The resistance value of the power source pull-up resistor 13 is set in such a way that the NPN type transistor 14 can sufficiently pull up the current with the normal power source voltage.
As described above, in the conventional ignition device for an internal combustion engine, the problem is encountered for example, when the surge generated at the time of switching or the like due to loads of other apparatuses is applied to the power source 6.
FIG. 15 is a waveform chart useful in explaining the operation of portions when a surge is applied. The description will hereinbelow be given with respect to the operation at timing when the ignition signal (a) is at a low level (OFF state) with reference to FIG. 15.
The current which is pulled up by the NPN type transistor 14 is determined on the basis of the voltage value of the power source 6 and the resistance value of the power source pull-up resistor 13. Then, when a surge is applied as shown in FIG. 15, the power source voltage (b) increases at timing of time t5 and hence a quantity of current pulled up by the NPN type transistor 14 increases.
Then, if the current has become unable to be pulled up sufficiently due to the insufficient ability of the NPN type transistor 14, then it becomes impossible to keep the collector voltage (switching signal)(e) of the NPN transistor 14 at a low level, which makes it impossible to reduce the gate voltage of the power device 5.
As a result, there is encountered the problem that the gate voltage increases synchronously with the surge and for a period of time ranging from time t5 to time t6, the malfunction occurs in which the level of the coil primary current (f) is made high (ON state) again.
There is encountered the problem that even if for example, the resistance value of the power source pull-up resistor 13 is made large in order to improve the above-mentioned malfunction, it is impossible to sufficiently drive the power source 5 during the low power source voltage of the power source as in the start-up.
In addition, there is encountered the problem that though increasing the size of the NPN type transistor 14 makes it possible to increase a quantity of pulled up current, the size of the chip increases accordingly.
Next, the description will hereinbelow be given with respect to the operation at timing when the ignition signal (a) is at a high level with reference to FIGS. 13 and 15.
When the surge is applied to the power source 6 at timing of time t2, in the high voltage cut-off circuit 11, the transistor 36 is turned ON through the Zener diode 30, and the resistors 34 and 35.
Since the transistor 36 is connected to the base terminal of the transistor 3, it cuts off the base signal (switching signal)(c) synchronously with the surge. The transistor 3 operates so that the input signal (switching signal) (d) of the drive circuit 4 is inputted to the base terminal of the NPN type transistor 14, and the waveform distortion of the collector voltage (e) of the NPN type transistor 14 is caused to cut off the gate signal of the power device 5.
As a result, there is encountered the problem that the malfunction occurs in which for a period of time from time t2 to time t3, the coil primary current (f) is cut off.
There is also encountered the problem that though in order to improve the above-mentioned malfunction, a capacitor provided outside the IC is included in the power source 6 to configure a filter circuit to absorb the surge, the number of components or parts increases, which leads to the increase in cost.